1. The field of art to which the invention pertains includes the art of internal conbustion engines and more particularly to such art as it applies to fuel and air induction systems therefor.
2. In by far the majority of currently used gasoline engines commercially marketed for automotive applications, fuel and air are metered and mixed by a carburetor connected to the intake manifold of the engine. Although these carburetors vary in detail from one manufacturer to the other, they are essentially similar and have over the years been regarded as generally satisfactory in supplying an adequate combustion mix to the engine.
Disclosed in cross reference application CR-1 is an improved apparatus for mixing fuel with air under substantially continuous sonic conditions and modulating the quantity of combustible fuel mix to meet operating demands of the engine with which it is utilized. Unlike a conventional carburetor, the apparatus of CR-1 generally comprises an elongated housing having a central passage defined in a venturi cross section intervening between the edge facings of opposite jaw faces supported in a fixed angular relation to each other. Varying the venturi flow area in accord with requirements is effected by changes to the passage in either a lateral or longitudinal plane. Air and fuel are received in an intake duct above the venturi throat at which sonic velocity is imparted to the mixture for achieving reduced emission of undesirable pollutants from the engine exhaust. A diffuser extanding immediately downstream from the throat efficiently recovers kinetic energy (velocity head) as pressure head and thereby enables sonic velocity at the throat to be maintained over substantially the entire operating range of the engine. Loss of sonic velocity through the throat occurs at low manifold vacuums (high absolute pressure) and is termed the "unchoke point".
While the device of CR-1 has been found to operate exceedingly well in meeting automotive emission standards established by governmental codes, some difficulty has been experienced in maintaining optimum engine performance throughout the operating range, particularly at idle and near idle conditions. Generally speaking, it has been difficult to maintain velocity of the mixture discharging from the diffuser exit to the engine manifold to within about 30 to 500 ft/sec. preferably to within about 100 to about 400 ft/sec. Exit velocities lower than minimum tend to recirculation and agglomeration of fuel drops in the diffuser while velocities higher than maximum tend to undesirable impaction of fuel drops in the intake manifold. In either direction away from the recommended range, degradation of mixture becomes increasingly worse resulting in a reduction in emission benefits.
Variation of flow area through the device of CR-1 from idle to fill throttle has been by lateral displacement of one or both mirror-like jaws of fixed length defining the venturi passage. In this arrangement, passage spacing at the venturi throat reduces at idle to an extremely narrow gap of high aspect ratio. For a 350 CID engine, the typical idle gap in this arrangement has been on the order of about 0.015 to 0.025 inches as compared to the gaps at wide open throttle (WOT) of between about 0.4 to 0.5 inches. Hence, with a constant diffuser length, i.e., measured from the throat to the exit plane in the direction of flow, the ratio of diffuser length to gap width is substantially greater at idle than at WOT. At the same time, area ratio between the exit and throat planes of the device is substantially increased at idle. Because of variation in area ratio with throttle opening a diffuser sized for WOT will incur a decreasing terminal velocity of the discharge mixture past the exit plane from several hundred feet/second to under 100 ft/sec. and even less than 30 ft/sec. at idle. Conversely, sizing the diffuser for idle operation can render the discharge velocity at WOT exceedingly high.